Exhaust after-treatment systems may be used to treat exhaust gases produced by a fuel burning engine. As one example, diesel engines may utilize an exhaust system that includes a selective catalytic reduction (SCR) system for reducing the amount of NOx that is ultimately discharged to the surrounding environment during operation of the engine. An SCR system may utilize the injection of a liquid reductant such as ammonia or urea into the exhaust gases where they may be mixed and absorbed onto a catalyst. The liquid reductant as it is evaporated and/or mixed with the exhaust gases can react with the nitrogen oxide (NOx) component of the exhaust gases to form water vapor and nitrogen gas.
The use of SCR systems in non-stationary or vehicle related applications can pose additional challenges due to geometric constraints. Some of these issues may be caused by an insufficient rate of evaporation and mixing of the injected liquid reductant with the exhaust gases as compared to the effective length of the mixing region. For example, where the liquid reductant is not sufficiently evaporated and mixed with the exhaust gases before reaching the catalyst, drops of liquid may be deposited onto the catalyst, which may leave residue upon evaporation and may eventually lead to degradation of the catalyst.
As set forth by the present disclosure, an exhaust system for an internal combustion engine for a vehicle is provided. The exhaust system comprises an exhaust passage for transporting exhaust gases from the engine; an injector coupled to a wall of the exhaust passage, said injector including an injection axis that is angled relative to a longitudinal axis of a mixing region of the exhaust passage; and a first mixing device arranged within the exhaust passage downstream of the injector within the mixing region, said first mixing device including a plurality of flaps, wherein said plurality of flaps are inclined relative to the longitudinal axis; a second mixing device arranged within the exhaust passage downstream of the first mixing device; wherein the injection axis of the injector intersects the first mixing device. As one example, the second mixing device may be configured as a helical mixer for increasing the distance of travel of the liquid reductant and exhaust gases flowing through the exhaust passage.
In this way, by utilizing the synergistic effects of the first mixing device for redirecting and increasing break-up of the liquid reductant and the second mixing device arranged downstream of the first mixing device, the reductant may be sufficiently mixed with exhaust gases produced by the engine before reaching a catalyst even where a relatively course spray is used by the injector, thereby reducing the amount of unmixed reductant deposited on the catalyst or walls of the exhaust passage and enabling a reduction in injector cost.